Hepatic fibrosis is examined in mice infected with schistosome species pathogenic for man. Mouse strains developed markedly different degrees of hepatic fibrosis following infection with S. mansoni. T cells are important for the formation of granulomas in both S. mansoni and S. japonicum infected mice, as determined from examination which subsets of T cells are involved in regulation of S. japonicum egg granulomas. BALB/c mice are not much more responsive to S. japonicum eggs than are BALB/c nu/nu mice, precluding studies of cell transfer in this strain, BALB/c mice do have more fibrosis than do their nude counterparts, emphasizing the role of T cells in hepatic fibrosis in this system and providing one more example of the dissociation of granulom size and fibrosis. T cells are required for the formation of normal granulomas, as indicated by marked differences between NCR outbred nu/nu versus nu/+ and between C57Bl/6 nu/nu versus C57Bl/6. Normal L3T4 cells or Ly2.2 cells from mouse spleen were transferred to C57BL/6 nude mice. Both cell types restored the formation of granulomas of normal size in mice subsequently infected with S. japonicum.